1. Field of the Invention
The present invention relates to solenoid operated hydraulic valves; and in particular to such valves in which pressure acting on an end of a spool creates a force that must be overcome by the solenoid in order to move the spool.
2. Description of the Related Art
Control valves have been developed for a variety of equipment to selectively apply and exhaust pressurized fluid to and from a component, the operation of which is controlled by that valve. As shown in FIG. 7, one such valve 200 has a spool 202, that slides within a bore 204 in response to operation of a solenoid actuator 206. The spool 202 opens to provide a path between a supply port 208 conveying pressurized fluid and a workport 210, which is connected to the component being operated by the valve. This provides pressurized fluid to the component being operated by the valve.
In many applications relatively high pressure acts on an end 212 of the spool and typically the entire cross sectional area of the spool. Some spools have a central blind bore 214 extending inwardly from that end. The workport pressure then acts on the annular end surface 216 and the parallel surface 218 at the inner end of the bore. In order to move the spool, the solenoid actuator has to overcome the force that results from the workport pressure acting on the combined valve element surface area 216 and 218.
As a consequence, if a larger spool is required to control the proper amount of flow, the larger surface area of the spool results in a greater feedback force. The greater feedback force in turn requires a greater counter force from a larger solenoid actuator. Therefore, it would be desirable to be able to increase the size of the spool to control a greater amount of flow without also having to increase the size of the solenoid actuator.